The present invention relates to improvements in the founding of iron and more particularly to improvements in sand molding media employed in forming molds into which molten iron is poured in the production of castings.
Founding is an ancient art in which a cavity is defined in a sand mold and then molten metal poured therein. After the metal cools, the cast article is removed, with the sand mold usually being broken up in the removal process. The usual and basic procedure for forming such sand molds is to compact a sand molding medium around a pattern and then to remove the pattern, leaving a cavity having the configuration of the pattern.
In order for the sand to maintain its molded, cavity defining configuration, it is necessary to provide a binding agent that will cause the sand particles to cohere. Clay has long been an accepted and suitable binding agent. Clay denotes a large group of hydrous alumino-silicate minerals. Individual mineral grains go down to microscopic size. When dampened, clay is tenacious and plastic. When dampened and then dried clay becomes permanently hard, particularly when dried at elevated temperatures.
The present invention is specifically directed to the founding of iron, where so-called green sand casting is a standard practice. This term denotes a process wherein molten metal is poured into a sand mold while it still retains the moisture that has been added to actuate the cohesive properties of the clay. Sand molding media for iron founding comprise three basic components, namely sand, clay and finely ground anthracite, commonly known in the trade as "sea coal". In use, a sand molding medium is moistened with water to provide a medium that is capable of being compacted around a pattern to form a mold cavity. After removal of the pattern, molten iron is poured into the mold cavity while the sand molding medium is still in its dampened or "green" condition. The sea coal, on and immediately adjacent the mold cavity surface, decomposes under the heat of the molten iron, as it is poured into the mold. A product of this decomposition is elemental iron, in the form of graphite, at the interface between the mold cavity and the poured iron. This elemental graphite serves the primary function of enabling the solidified casting to be released from the mold, free of sand particles. A secondary benefit of the elemental graphite is that it tends to level the surface of the mold cavity, thereby producing a smoother surface on the cast article.
In more recent times, it has been recognized that various advantages can be realized by substituting alternate carbon sources for a portion of the sea coal. Thus, there have been various proposals to employ bitumens, such as asphaltic emulsions, asphaltene chips, petroleum pitch and uintaite (a naturally occurring asphaltic deposit found in the Uinta Mountains in Utah and. available for the American Gilsonite Co., Salt Lake City, Utah, under the trademark Gilsonite). The use of such alternate, or secondary, bitumens is discussed in a paper, by the present inventors, published in the Journal of the American Foundrymans Society, Vol 95, pages 133-138 (Date of publication 1978).
At this point, it will be briefly noted that it is a well established practice of the trade for a foundry to purchase a "pre-mix", which comprises a clay component and sea coal. The foundry then mixes the "pre-mix" with sand from a local source to provide the sand molding media required for its operations.
At least one pre-mix having a supplemental bitumen has been commercially available for several years. This is to reference the use of an asphaltic (complex hydrocarbon) product which is derived from the distillation of petroleum. This petroleum based asphalt is employed in the form of an emulsion, i.e., the asphalt is a supercooled liquid in a highly dispersed emulsion. One advantage of this asphaltic emulsion is that it is richer in carbon than anthracite coal, that is, a lesser quantity of the asphaltic emulsion is required for a given quantity of sand molding medium.
It is to be noted that sand molding media employing either coal alone, or coal in combination with a supplemental carbon source, viz., the referenced asphaltic emulsion, have adequate strength characteristics for molding operations. This is to bring to attention the fact that the cohesive strength of the sand molding medium is most critical in its "green" condition, that is, when it is moistened. After being compacted to define a cavity, the "green" molding medium must have sufficient strength to withstand any forces incident to removal of a pattern, so that the cavity configuration is maintained intact. Next, sand molding media, when in a green stage, must have sufficient strength to withstand the forces incident to the mold being moved and repositioned in various fashions in the process preparing it for the pouring of metal into the cavity. Further, the sand molding media must have sufficient cohesive strength to withstand the hydraulic forces incident to pouring molten iron into the cavity.
Drying of a "Green" mold occurs extremely rapidly and can occur while the metal is still molten and continues to exert hydraulic forces on the mold structure. The dry strength of the molding medium is therefore critical in assuring that the integrity of the mold will be maintained to the end of obtaining cast articles of the proper configuration.
It will be briefly mentioned that there is another significant, objective characteristic of sand molding media, namely, permeability. A relatively high permeability is required in order to prevent damage to the mold when molten iron is poured into the mold cavity. This is to point out that when molten metal is poured into the mold cavity, air must be displaced though the mold medium. More importantly, because the sand molding medium is damp, steam can be generated in a rather violent, or explosive, fashion. Such steam must be vented through the molding medium with a minimum of gas flow resistance. All of this requires a porous mold structure having a relatively high gas permeability.
Strength characteristics and permeabilities are capable of objective determination and acceptable green and dry strengths for sand molding media, as well as permeabilities, are now established.
After an item has been cast, the sand mold is broken up and then recomminuted for reuse. Over a period of time it becomes necessary to add fresh amounts of the clay and carbon additive. Similarly, it is a common practice to also add fresh sand. This not only maintains a more or less constant ratio of the sand, clay and carbon components, but also compensates for the accumulation of ash that is a byproduct of the decomposition of the sea coal.
The referenced pre-mix, that includes an asphaltic emulsion, has found acceptance because of several advantages. Primarily these advantages are found in the ability to minimize costs by the use of less pre-mix and/or by reducing the total amount of carbonaceous material in the pre-mix. Further, it was demonstrated that the amount of additional, "make-up" pre-mix, needed in recycling a sand molding medium, was reduced. Additionally it was demonstrated that this hybrid hydrocarbon pre-mix gave improved compactability, which facilitated forming molds, as well as minimizing the number of faulty castings. These advantages were attained, while at the same time maintaining the necessary, minimum green and dry strengths. Also, the gas permeability characteristics were sufficient to properly vent the molds when the molten iron was cast.
Another factor to note is that as green sand molding medium is compacted around a pattern (in the normal case) to form a mold cavity. The characteristics of the sand molding medium can have a great impact on the "workability" of the medium and the ability to compact, viz., densify, the medium and also the ease with which densification can be attained. This factor is relevant to the fact that both the green strength and dry strength of a sand molding medium are directly proportionate to the density of the sand molding medium after it has been compacted to define a mold cavity. There is thus a preference for sand molding media that have a workability characteristic which facilitates obtaining a desired, relatively high and consistent density of the compacted molding medium. While the "workability" characteristic is subjective, it is, nonetheless, a recognized standard for sand molding media.
The referenced, petroleum based, asphaltic emulsions have a limited number of commercial applications, or uses, beyond its use in sand molding media. Therefore, there is little stimulus for widespread availability of this product. Thus there is an absence of price competition. For this and other reasons, such emulsions can be difficult to obtain in desired quantities and, in all events, are relatively expensive. A further shortcoming with this asphaltic emulsion is that the petroleum derived asphaltic product emits benzene, when it decomposes under the heat of molten iron. While there has been no demonstrated hazard from such benzene emissions, benzene is deemed undesirable, if not harmful, in many situations. Recognizing that governmental regulations are often imposed where no realistic hazard exists, it is prudent and desirable to greatly minimize, if not eliminate benzene emissions from this aspect of foundry operation.
The present invention focuses on pre-mixes employing sea coal and an alternate carbon source. In a more specific sense, the invention seeks to overcome problems and shortcomings associated with employing emulsions of petroleum based asphalt.
A primary object of the invention is to provide a supplemental, carbon rich, hydrocarbon source for sand molding media employed in the founding of iron.
Another object of the invention is to achieve the foregoing end and, additionally, to greatly minimize, if not eliminate the emission of benzene during decomposition of the hydrocarbon in the casting process.
Yet another object of the invention is to achieve the foregoing ends in a manner that preserves the necessary characteristics of a sand molding medium for use in iron founding.
A further object of the present invention is to attain the foregoing ends and, additionally, to further improve the facility with which sand molding media can be densified to thereby provide increased strength for the sand molding medium on a more consistent basis.